The present invention relates to the preparation of multilayered plastic preforms and containers including a barrier layer.
Multilayered plastic preforms and containers are well known and advantageous, see for example, U.S. Pat. Nos. 3,869,056, 4,149,645, 4,092,391 and 5,464,106.
A conventional method for making the liners for the multilayered articles is by preparing a multilayered laminate and thermoforming the laminate into a tubular article, followed by coating the tubular article to form a tubular preform. The multilayered tubular preform may then be converted to a multilayered container by blow molding.
The multilayered laminate may include a barrier layer made of a thermoplastic barrier material, as polypropylene (PP), ethylene vinyl alcohol (EVOH), polyethylene naphthalate (PEN) or an acrylonitrile copolymer, for example.
The processes available to effect conversion of a multilayered laminate into a liner result intrinsically in disadvantageous differences in the wall thickness of the liner. Thus, the neck portion of the liner tends to be thicker than the body portion because that part of the tube or sheet from which the neck portion is made undergoes less stretching than the body portion. Similarly, the bottom portion of the liner tends to be thick if it is the location of a tube's welded end, with a near double thickness, or because the bottom portion of a sheet remains at original gage. Control of the barrier thickness is at best extremely difficult, and it is nearly impossible to obtain optimal thickness distribution for the barrier, which in most instances requires the least barrier thickness at the neck and the most in the body portion. Typically, the barrier is a part of the liner, and the liner is typically made from a laminated tube or sheet, and the barrier is therefore one of the layers therein.
An example of a barrier to the migration of CO.sub.2 or O.sub.2 is EVOH, but it is cost-effective only if it is protected from moisture because its permeation resistance decreases sharply as it absorbs moisture. Thus, an exemplary liner for a container for water-based liquids may be composed of a laminate with a polypropylene (PP) layer to protect the next layer of the gas barrier EVOH, followed by yet another layer, for example of polyethylene terephthalate (PET), all of it perhaps molded from a tube or sheet.
Yet another disadvantage to present practices is the practical limitation for the barrier component of the liner to be easily formable and of relatively inexpensive materials, typically thermoplastics. Thus, excellent barrier materials that are thermosetting, e.g. polyepoxides and expensive thermoplastics, e.g. liquid crystal polymers (LCPs), and the inorganic oxides of silicon, SO.sub.x, find no practical use in liners. The epoxides are now used as barriers for PET bottles, but in a manner that is inconvenient, expensive and environmentally undesirable, the LCP's are prohibitively expensive if used in quantities dictated by current methods of application to rigid containers, and SiO.sub.x not at all.
It is, therefore, a principal object of the present invention to provide an improved process for forming a multilayered plastic container, and an improved multilayered preform and multilayered container.
It is a further object of the present invention to provide a process, preform and container as aforesaid including a barrier material where the barrier material varies in thickness.
It is a still further object of the present invention to provide a process, preform and container as aforesaid which uses polyepoxides, LCP's or inorganic glasses, such as SiO.sub.x, as barrier materials.
It is an additional object of the present invention to provide a process, preform and container as aforesaid which is inexpensive and commercially feasible compared to current methods.
Further object and advantages of the present invention will appear hereinbelow.